This invention relates to an improved extruder screw for use in an extrusion apparatus for working a wide range of solid plastic materials into a substantially homogeneous, molten state suitable for formation into any desired shape by extrusion or injection into a die or mold. More particularly the improved extruder screw, hereinafter, for brevity, referred to as screw, of the present invention is most readily used in what is known as the single screw extruder.
Extrusion, injection molding or blow molding with a single screw extrusion machine or apparatus includes feeding the solid plastic or polymeric material in pellet, chip, powder or flake form to the inlet end or feed section through a hopper mounted in an opening in the top side at or near the drive end of a heated barrel in which the screw is rotatably mounted. The screw includes at least one helical thread integrally mounted or formed on the core to create a channel, down which the plastic is moved downstream from the inlet end to the discharge end by forces exerted by the rotation of the screw.
In the beginning of the art of extrusion, screws having uniform pitch and uniform thread depth were used. It soon became apparent that such screws were very inefficient in bringing the solid plastic material to the desired homogeneous molten condition. Many variations were tried to overcome the problems encountered in trying to improve the technique of extrusion and the very closely related technique of injection molding and blow molding. Some of these variations involved change of pitch, change of channel depth between adjacent threads, providing different sections along the length of the screw where pitch and/or channel depth were varied from section to section. Although these variations did provide significant improvement in the quality of the articles formed by extrusion or injection, hereinafter included within the term extrusion, further improvement was desired.
A review of the more recent art shows that in order to improve the homogenity of the melt, A. J. Palfey, in U.S. Pat. No. 3,023,456, filed Aug. 3, 1959, feeds molten material from another source to an extruder having a screw with varying root diameter along its length to improve mixing of the molten material as it is forced down the screw.
Maillefer, S. A., a Swiss corporation, was issued British Pat. No. 964,428, published July 22, 1964, for an extruder screw having a constant core diameter that provides several means for separating the molten material from the solid material. One of the means provides a central passage along the axis of the screw into which the molten material flows through small holes in the land of the thread from the main feed channel. Another means uses two threads with different pitch and diameter. The pitch of each thread is such that the second thread originates out of the first or feed thread near the inlet end of the screw where the solid material is fed to the screw and again merges with the feed thread just prior to the discharge end of the screw. The second channel created by the second thread widens downstream while the first or feed channel narrows. The feed thread has a diameter that provides the normal clearance between the thread and the heated barrel while the diameter of the second thread is slightly smaller so that only the molten material can flow over the land of the thread. In the preferred embodiment the feed thread has less pitch so that as molten material is formed it flows over the second thread of less diameter and greater pitch.
A similar type of extruder screw is used by P. Geyer in U.S. Pat. 3,375,549, filed Apr. 5, 1961 for refining and separating plastic material, more particularly, for refining cured or scorched rubber compounds and for separating larger hard particles therefrom. Because of the blind end created by the merging of the two threads, any solid material at the discharge end of the screw is trapped. To remove this trapped solid material, Geyer provides a weep hole at the discharge end which permits the discharge of the solid material separately from the molten refined material.
F. K. Lacher, in U.S. Pat. No. 3,271,819, filed Feb. 26, 1962, disclosed another type of extruder which is similar to Geyer and Maillefer. His screw has a feed section, a transition section and a metering section, and is provided with means for permitting restricted communication between the feed channel and the second channel such that the flow of large particles of unplasticized material from the feed channel to the second channel is restricted. The root diameters of both the original feed channel and the second channel are increased progressively along the transition section.
M. A. Natov, et. al., in U.S. Pat. No. 3,504,400, filed Oct. 23, 1967, are not concerned with the separation of molten material from solid material, but with the providing of resistance to back-flow of plasticized or molten material. They accomplished this by providing a second thread of different pitch intermittently along the first thread. The second thread is of less diameter and makes no contact with the first thread. Although the difference in pitch of the two threads causes some material to flow from one channel to the other, molten material is not separated from solid material at the start or end of the second thread.
H. Schippers et. al., in U.S. Pat. No. 3,701,512, filed Apr. 7, 1971, use a pair of parallel threads of equal pitch. The second thread starts downstream of the first thread in the center of the channel created by the first thread to create two channels of equal cross-sectional area. The diameter of the second thread is such that its clearance with the heated barrel is at least twice that of the first thread. The second thread acts as a shearing screw as material, either solid or molten, is forced to flow over it by inversely varying the root diameters of the two channels. Like Natov et. al., solid material as well as molten material can be present in both channels since no separation is provided at the start or end of the second channel.
R. A. Barr, in U.S. Pat. No. 3,698,541, filed Aug. 11, 1971, provides such separation. He accomplishes this by using a tapered, forward widening of the feed thread at the end of the feed section. At the start of the next section, which he calls the melting section, the diameter of the leading edge of the widened thread is progressively decreased to the desired diameter of a second thread so that only molten material can flow over the second thread. The undercutting used to effect this decrease in diameter is then continued in such a way as to create the second thread with a constant diameter and a constant pitch but with a subchannel behind it whose root diameter progressively decreases along the melting section. At the same time the root diameter of the original feed channel progressively increases along the melting section to the diameter of the second thread causing the second thread to disappear near the end of the melting section. At the start of the next or transition section, the root diameter of the subchannel is rapidly increased while the root diameter of the original feed channel is rapidly decreased so that the root diameters of the two channels become equal to provide a metering channel of a constant root diameter.
In U.S. Pat. No. 3,858,856, filed Jan. 26, 1973, J. S. Hsu uses an extruder screw similar to Barr but initiates the second thread by a different means. Instead of widening the feed thread in the forward direction, he widens it in the backward direction and immediately decreases the diameter of the leading edge to form a second or barrier thread and starts a second channel behind the barrier thread. The original diameter of the feed thread is maintained on the trailing edge of the widened thread to form a main thread which continues to the end of the screw. In this way, Hsu narrows the feed channel one turn ahead of that of Barr. Another difference is that the barrier thread, the one with the smaller diameter, is a continuation of the feed thread in Hsu's screw whereas in Barr's screw the barrier thread is offset forwardly and not a continuation of the feed thread.
While both screws of Barr and Hsu do provide means for separating the molten material from the solid material, they do it at the expense of narrowing the feed channel down which the solid material is being forced to move by the rotation of the screw, thus impeding the smooth flow of the solid material down the channel. It would be highly desirable to eliminate this impediment of flow.
It is an object of this invention to obtain an efficient and uniform melting of solid plastic material during preparation thereof for extrusion with the least possible interference to the material flow in the screw channel.
Objects ancillary to the foregoing are to teach and define a method to accomplish said objective.
Another object is to provide a method and apparatus for progressively melting solid plastic material in such manner as to establish maximum heat transfer area between the heated barrel of an extruder and the solid material.
A further object is to provide an extruder screw in conformity with the above-cited objects which is easy to manufacture.
It is another object to reduce the power input and thus to reduce the temperature of the molten material as a result of lower shear energy inputs in conveying the molten material in a melt channel separated from the solid material in a solid channel.
Another object, which can be used in conjunction with the above stated objects, to even further ensure homogenity of the discharge molten material, is to provide means for a second transfer of the molten material over a barrier thread inside the extruder.